All vessels, not built in the US must have import duty (1 1/2%) paid to the US government to be offered for sale to US residents in US waters. This is paid once, and transfers to subsequent buyers, unless the vessel is sold to a buyer who registers outside of the US at any point. Then the duty becomes payable again. Us Built vessels are not subject to this duty, again, unless they are registered outside of the US.
If a US resident purchases a boat that has not had the import duty paid -only an offshore purchase is legal, He must then pay the import duty if he wishes to register the boat in the US. Or he can register the boat outside of the US and must enter US waters under a cruising permit.
A US flagged vessel can remain outside of US waters, and not be responsible for the import duty. However, any vessel owned by a US citizen, and US flagged , must pay the import duty upon entering US waters.
This is the same principle as if a US citizen went to a foreign country, bought and expensive Rolex watch, and then brought the watch into the US - Import duty is due immediately on the watch upon entry.